diff --git a/src/scribe/mcp/tools/family.py b/src/scribe/mcp/tools/family.py
index 30e25e2d..23b3a35a 100644
--- a/src/scribe/mcp/tools/family.py
+++ b/src/scribe/mcp/tools/family.py
@@ -33,7 +33,8 @@ async def list_family_ideas(
 
 async def get_family_idea(note_id: int) -> dict:
     """One family idea with what an evaluation needs: its state, platforms,
-    full decision history, ledger counts, THE THREE CRITERIA, the `ledger`
+    decision history (brief — each reason once; list_family_decisions has
+    the full rows), ledger counts, THE THREE CRITERIA, the `ledger`
     (every project's answer, with `needs_recheck`), and the `precedents` —
     the decisions on the ideas nearest this one by meaning.
     Read the precedents before deciding, and decide consistently with them
diff --git a/src/scribe/models/family.py b/src/scribe/models/family.py
index d89bba83..72202387 100644
--- a/src/scribe/models/family.py
+++ b/src/scribe/models/family.py
@@ -357,3 +357,37 @@ class FamilyDecision(Base, CreatedAtMixin):
             "user_id": self.user_id,
             "created_at": iso(self.created_at),
         }
+
+    def to_brief(self) -> dict:
+        """The decision as a reader WEIGHING it needs it: what was decided,
+        why — once — and the state it moved between.
+
+        Precedents and an idea's history are read in bulk, a handful per
+        call, and the full row repeats itself: an assessment's reason is
+        stored again inside `after` (and the prior one inside `before`), and
+        a promotion's evidence carries the whole criteria reasoning. So a
+        state here is its status, version and platforms only, and evidence is
+        the cited list alone (#5131). The full row is `to_dict`, which the
+        decision log serves.
+        """
+        return {
+            "id": self.id,
+            "idea_id": self.idea_id,
+            "project_id": self.project_id,
+            "action": self.action,
+            "reason": self.reason,
+            "before": _brief_state(self.before),
+            "after": _brief_state(self.after),
+            "evidence": list((self.evidence or {}).get("evidence") or []),
+            "created_at": iso(self.created_at),
+        }
+
+
+# What a decision's before/after keeps in its brief: the state, not the prose.
+_BRIEF_STATE_KEYS = ("status", "canon_version", "platforms")
+
+
+def _brief_state(state: dict | None) -> dict | None:
+    if state is None:
+        return None
+    return {k: state[k] for k in _BRIEF_STATE_KEYS if k in state}
diff --git a/src/scribe/services/family.py b/src/scribe/services/family.py
index 0f328802..e0ed02ad 100644
--- a/src/scribe/services/family.py
+++ b/src/scribe/services/family.py
@@ -211,7 +211,8 @@ async def get_idea(user_id: int, note_id: int) -> dict | None:
             "platforms": await _platform_slugs(session, note_id),
             "topic": await _topic(session, user_id, idea.topic_id, rules=True),
             "adoptions": counts,
-            "decisions": [_decision_dict(d) for d in decisions],
+            # Brief: the history is read in bulk; the full rows are list_decisions (#5131).
+            "decisions": [d.to_brief() for d in decisions],
             "undoable_decision_id": _undoable_id(decisions),
         })
     out["criteria"] = list(CRITERIA)
@@ -383,8 +384,8 @@ async def precedents(user_id: int, note_id: int, limit: int = 5) -> list[dict]:
     for score, n in ranked:
         d = decisions.get(latest.get(n.id))
         if d is not None:
-            item = _decision_dict(d, n.title)
-            item["similarity"] = round(float(score), 3)
+            item = {**d.to_brief(), "idea_title": n.title,
+                    "similarity": round(float(score), 3)}
             out.append(item)
     return out
 
diff --git a/src/scribe/services/family_adoption.py b/src/scribe/services/family_adoption.py
index e4313fb3..435b1a84 100644
--- a/src/scribe/services/family_adoption.py
+++ b/src/scribe/services/family_adoption.py
@@ -545,8 +545,7 @@ async def assessment_precedents(user_id: int, project_id: int, idea_id: int,
     for d, idea_title, project_title in rows:
         if not await access.can_read_project(user_id, d.project_id):
             continue
-        item = d.to_dict()
-        item.update({"idea_title": idea_title, "project_title": project_title})
+        item = {**d.to_brief(), "idea_title": idea_title, "project_title": project_title}
         if d.idea_id == idea_id:
             item["relation"] = "same idea, another project"
             out_same.append(item)
diff --git a/tests/test_family_models.py b/tests/test_family_models.py
index 050d17df..c45ec23d 100644
--- a/tests/test_family_models.py
+++ b/tests/test_family_models.py
@@ -67,3 +67,47 @@ def test_a_canon_ideas_scope_lives_on_the_idea_and_nowhere_else():
 
     assert "topic_id" in family.FamilyIdea.__table__.c
     assert not {c.name for c in RulebookTopic.__table__.c} & {"platform_id", "platform_ids"}
+
+
+# --- a decision read as precedent (#5131) ------------------------------------------
+
+def _assessment():
+    reason = "the APK is built in CI and signed with the one release key"
+    return family.FamilyDecision(
+        id=7, idea_id=3, project_id=5, action="assess", reason=reason,
+        before={"status": "unassessed", "reason": "", "canon_version": None},
+        after={"status": "adopted", "reason": reason, "canon_version": 2},
+        evidence={"evidence": ["ci.yml::android-release"], "source": "shapes"},
+        precedent_ids=[4, 6], decided_via="agent", user_id=1,
+    )
+
+
+def test_a_brief_carries_the_reason_once():
+    brief = _assessment().to_brief()
+    assert brief["reason"].startswith("the APK is built")
+    assert brief["before"] == {"status": "unassessed", "canon_version": None}
+    assert brief["after"] == {"status": "adopted", "canon_version": 2}
+    # Nothing in it says the reason a second time.
+    assert repr(brief).count("the APK is built") == 1
+
+
+def test_a_brief_keeps_the_cited_evidence_and_drops_the_rest():
+    promotion = family.FamilyDecision(
+        id=8, idea_id=3, project_id=None, action="promote", reason="proven twice",
+        before=None,
+        after={"status": "canon", "applies_when": "an app that ships its own APK",
+               "canon_version": 1, "platforms": ["android-app"]},
+        evidence={"criteria": {"proven": "a long paragraph"}, "evidence": ["#12"],
+                  "ledger_rows_opened": 3},
+    )
+    brief = promotion.to_brief()
+    assert brief["evidence"] == ["#12"]
+    assert brief["before"] is None
+    assert brief["after"] == {"status": "canon", "canon_version": 1,
+                              "platforms": ["android-app"]}
+
+
+def test_the_full_row_is_still_the_audit_shape():
+    full = _assessment().to_dict()
+    assert full["after"]["reason"] == full["reason"]
+    assert full["evidence"]["source"] == "shapes" and full["precedent_ids"] == [4, 6]
diff --git a/tests/test_integration_family_adoption.py b/tests/test_integration_family_adoption.py
index 8c911784..dda4a712 100644
--- a/tests/test_integration_family_adoption.py
+++ b/tests/test_integration_family_adoption.py
@@ -236,6 +236,9 @@ async def test_another_projects_answer_is_recorded_as_precedent(family):
     assert first["decision"]["id"] in second["decision"]["precedent_ids"]
     assert second["precedents"][0]["relation"] == "same idea, another project"
     assert second["precedents"][0]["project_title"] == "android one"
+    # A precedent says its reason once: before/after are state only (#5131).
+    assert second["precedents"][0]["reason"] == "the source app"
+    assert "reason" not in second["precedents"][0]["after"]
 
 
 # --- recheck --------------------------------------------------------------------------